Fast neutron irradiation affects the properties of beryllium oxide by causing displacements and by causing nuclear transmutations. This report outlines the overall aims of a programme to investigate this problem, reviews the information from overseas laboratories, and describes the results obtained to date at Lucas Heights. Results are given of measurements of properties of beryllium oxide fabricated by various methods and irradiated to doses of up to 7 x 10 20 nvt (fission neutrons) at temperatures of 75 — 700ºC. The properties include macroexamination, dimensions, porosity, lattice parameter and line broadening, mechanical properties, thermal conductivity, metallography, and long wavelength neutron scattering. It is shown that an anisotropic lattice growth occurs which results in crumbling of the material at high doses. The damage rate is much smaller for irradiation at 500 — 700ºC than for equivalent doses at 100ºC. Fine—grained (<3/μ) material withstands crumbling up to much higher doses than coarse—grained material. The relationship between macroscopic growth, lattice growth, and the cracking and powdering is discussed in some detail and the results used to show the reasons for apparent discrepancies in data from overseas laboratories. Information relating to the defect structure is discussed and it is suggested that interstitial clusters in the basal planes are probably the cause of the marked anisotropy in the lattice growth. The effect of neutron energy spectrum on the damage rate is discussed and finally the potential of beryllium oxide as a reactor material is assessed. It is concluded that very fine grained material should withstand doses of at least 1— 2 x 10 21 nvt at temperatures of 500 — 1000ºC without serious deterioration of properties. More information, particularly on changes of mechanical properties and thermal conductivity, is required to confirm this conclusion and to ascertain whether the material will withstand higher doses.